1. Field of the Invention
The present invention is related to an optical fiber transmission equipment, and particularly to an optical fiber transmission equipment using an optical wavelength division multiplexed signal, which can increase the transmission capacity.
2. Description of the Prior Art
Since the optical fiber transmission equipment using an optical wavelength multiplexed signal can increase its transmission capacity without changing the transmission path, it is a technique which is expected to be applied to the future optical fiber transmission equipment. If an optical wavelength multiplexed signal is utilized for an optical fiber transmission system which uses an optical amplifier as an optical repeater, in addition to the degradation factors in the optically amplified fiber transmission system (e.g. the optical waveform distortion due to the optical fiber chromatic dispersion, the optical signal-to-noise ratio degradation due to the accumulation of the amplifier noise, the distortion due to the nonlinearity of the optical fiber), there are other degradation factors such as the distortion due to the crosstalk between optical signals of different wavelengths, the distortion due to the four-wave mixing between the signal, and soon. Thus, the maximum transmission distance and the maximum channels of the WDM signal, which are the characteristics of the optical fiber transmission system, are limited. The distortion due to the four-wave mixing between the signal means the phenomenon in which, if the frequencies of two signal lights to be multiplexed are f1 and f2 (f1&lt;f2) for instance, lights having frequencies of f1-(f2-f1) and f2+(f2-f1) are generated by mixing the two frequencies, and a distortion is caused by the interference between the light generated by the mixing and the signal light.
Specifically, in the wavelength division multiplexed optical fiber transmission system for a long distance of about 1,000 km or longer, which uses an optical amplifier as a repeater, the effect of the four-wave mixing is dominant constraint factor of the system. As the reason for that, the following two points can be given. (1) The generation efficiency of four-wave mixing is proportional to the light intensity of each signal pertinent to the generation of the four-wave mixing, and inversely proportional to the absolute value of the optical fiber chromatic dispersion which is the transmission medium for the light signal (reference: N. Shibata et al., Electronics Letters, vol. 24, pp. 1528-1529, 1988). (2) In a transmission path in which a multiplicity of optical amplifiers are relayed, the four-wave mixing produced in the respective repeater sections are added together, accumulated and increase.
As a future optical fiber transmission system, a system is proposed in which, to minimize the waveform distortion due to the optical fiber chromatic dispersion thereby to increase the transmission bit-rate of a light signal, the absolute value of the fiber chromatic dispersion of the optical fiber for transmission becomes substantially zero in the wavelength band of the optical signal to be used. In this optical fiber transmission system, the distortion due to the four-wave mixing which is caused by the transmission of an optical wavelength multiplexed signal would be more dominant than other distortion factors for the above reason.
To suppress the distortion due to the four-wave mixing, it is necessary to separate the signal wavelength of the wavelength multiplexed signal light and the zero-dispersion wavelength of the transmission path from each other. As an example of this, a method for designing a wavelength multiplexing system was proposed (reference: Ishikawa et al., the Spring National Conference of the Institute of Electronic Information and Communication Engineers of 1994 SB-8-6). In this method, the signal waveform distortion due to the chromatic dispersion produced by the separation of the signal wavelength of the wavelength division multiplexed signal light and the zero-dispersion wavelength of the transmission path is compensated by a dispersion compensator. This signal waveform distortion can be reduced by such compensation.
However, in this conventional method, the signal wavelength of each signal light is determined in a particular system length from the gain bandwidth of the optical amplifier, or the like, and there was a problem that system length change could not be served.